In vehicles with a thermal engine, using the heat given off by the engine to heat the vehicle passenger compartment is known. In vehicles with an electric motor, the heat given off by the electric machine used to drive the vehicle is too small to fulfill such a function. An identical problem is posed, even if this is to a lesser degree, in hybrid vehicles, that is to say with a both thermal and electric drive.
To solve this problem, it has already been proposed to operate air-conditioning loops reversibly. They are thus configured so as to introduce alternately cold air or hot air into the passenger compartment, according to the request from the user.
They use a heat exchanger, situated at the front face of the vehicle, so as to be swept by an airflow at ambient temperature passing through the radiator grille. Said exchanger serves to condense the refrigerant fluid circulating in the air-conditioning loop when said air-conditioning loop is used to cool the passenger compartment and to evaporate said fluid in the opposite case, that is to say when the air-conditioning loop is functioning as a heat pump to heat the passenger compartment.
The thermal performance of such heat exchangers is difficult to optimise since the solutions for improving the functioning thereof as a condenser are generally opposed to those for improving the functioning thereof as an evaporator.
More precisely, in condensers or evaporators of the interlayered tube type, it has been known for a long time that it is advantageous to circulate the refrigerant fluid serially in passes containing a given number of tubes. In condensers, it has also been known for a long time that decreasing the number of tubes from one pass to another optimises the heat exchange while limiting losses of pressure. Persons skilled in the art also know that such a distribution of tubes is on the other hand unfavourable to the functioning of evaporators.
A first solution for avoiding this situation is to reverse the direction of circulation of the fluid in the heat exchanger, but such a solution increases the complexity of the air-conditioning loop.
For heat exchangers that are to serve alternately as condenser and evaporator, without reversal of the direction of circulation of the refrigerant fluid in the heat exchanger, a person skilled in the art is then naturally led to propose heat exchangers having a configuration that is as symmetrical as possible in order to avoid being detrimental to one operating mode with respect to the other. In the case of heat exchangers of the interlayered tube type with a plurality of passes, this results in the use of two passes, having an identical number of tubes per pass or at the least that remains similar from one pass to another.
This being the case, a particularly critical problem is the risk of icing of the heat exchanger in heat pump mode. The appearance of such a phenomenon tends to stop all or some of the heat exchange because of the increase in the loss of air pressure. The degradation of the heat exchange due to the icing tends to reduce the evaporation temperature and the pressure of the refrigerant fluid inside the heat exchanger, which increases the risk of icing of the exchanger accordingly.
Another particularly critical problem relates to the loss of pressure internal to the heat exchanger. In evaporator mode, it is known that the density of the refrigerant fluid is lower than in condenser mode, which has the effect of increasing the loss of pressure. It thus appears essential to seek to reduce the loss of pressure in evaporator functioning in order to improve the thermal performance.
To avoid such a risk, the use of a smaller number of tubes in the first pass has already been considered, while placing this first pass in the bottom portion of the exchanger, the exchanger being positioned in a substantially vertical plane and the tubes being oriented substantially horizontally.